Energy storage using large superconducting magnets has been proposed for leveling daily load requirements on electrical utility systems. Excess energy generated during off-peak hours can be stored and later returned to the power grid during high demand periods. By connecting the superconducting energy storage magnet to the power system with a bridge-type inverter, it is possible to obtain very efficient energy transfer between the storage magnet and the power system, as more fully described in U.S. Pat. No. 4,122,512 to Peterson, et al.
The large energy storage magnets proposed for storing sufficient energy to allow load leveling on a power grid utilize multiple turns of composite normal and superconducting material. The current flowing in the turns of the magnet naturally produces a net magnetic field and any conductor in the field will experience a force at each point on the conductor oriented at right angles to the current and the magnetic field. Since superconducting magnets of the size proposed for electrical system energy storage will conduct extremely large currents and will generate strong magnetic fields, the forces experienced by the conductors will be very large. If the turns of the magnet coil were formed as conventional circular turns, and were unsupported, the tension at any cross-section in the conductor would be equal to BIR, where B is the component of the magnetic field experienced by the conductor perpendicular to the plane of the conductor (axial magnetic field), I is the current in the conductor, and R is the radius of curvature of the turn. In the large energy storage magnets under consideration, all of these factors will be very large, e.g., several hundred thousand amperes will be conducted in a field of several teslas in a solenoid magnet having a radius which may be several hundred meters. Since no conductor by itself could possibly withstand the forces that would be exerted on the conductor under these conditions, an external support structure capable of resisting the large loads imposed on the conductor is necessary.
One approach to the problem of adequately supporting a superconducting magnet is shown in U.S. Pat. No. 3,980,981 to Boom, et al. The structure disclosed in that patent includes a rippled composite superconducting-normal conductor which is laid out in a single layer of turns disposed in a trench formed in the ground. Each ripple in the conductor lies in a plane normal to the net magnetic field experienced by that conductor. The outward force on the conductor is opposed by support columns which engage the conductor at its innermost portions between the ripples. The supporting columns extend radially to an outer support wall which may be formed in bedrock. The columns can be made of insulating material so that the necessary thermal shielding Dewar is accommodated around the conductor with minimal interference from the radial support members.
The single layer magnet coil disclosed in U.S. Pat. No. 3,908,981 has several advantages, including ease of maintenance since both sides of the conductor are readily accessible, a simple construction for the conductor, reduced stress resulting from the rippling in the conductor, accessibility of both sides of the conductor with a mechanical shorting switch to protect against failure of the cooling system, ability to surround the conductor with superfluid helium for maximum cooling efficiency, and low voltage difference levels between turns in the magnet coil. Despite the advantages of the single layer design, all of the current circulating in the superconducting coil must be carried by a single conductor. For magnet designs under consideration for power system load leveling, a current capacity of 750,000 amperes or more would be carried by the single conductor. Additionally, the resultant of the forces on the rippled conductor will be substantially radial, so that a very strong and stable outer support mass is required to carry the loads that will be imposed when the superconductor is carrying current. If the conductor is buried in and surrounded by bedrock, which is intended to carry these radial forces, the bedrock must have reasonably good structural integrity and be stable over time.
The superconducting energy storage magnet shown in U.S. Pat. 4,622,531 to Evssa. et al. has inner and outer coils which are supported and restrained by support structure. The coils experience primarily inwardly directed (toward the supports between the coils) forces. The net outward force on the rippled conductors of the magnet is transferred outwardly on support struts to the walls of the trench in which the superconducting magnet is formed.
For superconducting magnets which are to be formed in trenches or tunnels in the ground, the surrounding soil or bedrock forms part of the support structure which absorbs the outward forces on the superconducting magnets. Magnets in the form of solenoids which have a low aspect ratio (height/diameter less than 0.1) have the advantage that relatively low pressure is applied to the face of the trench in the rock or soil. This advantage allows such low aspect ratio coils to be mounted closer to the ground surface where the soil or rock is ordinarily relatively weaker than the rock surrounding a tunnel formed in deep bedrock. However, the soil in the face of a trench at the top of the trench is generally weaker and less able to carry the radial pressure supplied by the superconducting magnet then the soil or rock at the bottom of the trench. The ripple conductor magnet structure shown in U.S. Pat. Nos. 3,980,981 and 4,622,531 has significant advantages for large energy storage magnets. Nonetheless, the composite conductors in such magnets will experience expansion strains as the magnet is charged and discharged and the current flowing through the magnetic increases and decreases. The "working" of the normal metal of the composite conductor due to the slight bending of the composite conductor that occurs during the charge-up and discharge of the magnet will, over time, increase the effective resistance of the normal metal. This is an important factor with a composite conductor formed utilizing high purity aluminum as the normal conductor because high purity aluminum is particularly susceptible to an increase in resistance with working.